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On commercial airplanes there are three main reasons for installing a parking brake function, as listed below:
Regulations require a means of holding the airplane with the brakes during emergency evacuation of the passengers and crew. This requires that the brakes stay applied for a minimum of 5 minutes with sufficient force to stop the airplane from rolling on a sloping runway.
Secondly, the convenience of having the airplane parked without the need for wheel chocks. Most operators of commercial jetliners require the parking brake to hold the airplane for a minimum of 18 hours. Commercial pilots will sometimes set the parking brake when the airplane is standing in traffic waiting for the runway to become available.
The third reason is to provide short-term measure for holding the airplane while chocks are put in place.
Long Term Parking
For long-term parking, there are two approaches that are commonly used for applying a parking brake:
The first method is most often used for conventional brake systems where the brake pedals are connected to the brake-metering valve through a mechanical linkage or cable system. We call this the pedal latching approach.
The second method is used on brake-by-wire systems where the brake pedals transmit an electrical signal to a control unit that actuates the brake pressure control valves. This second method can also be used for conventional brakes. It uses a system of valves to apply pressure to the brakes independently of the brake pedals. We call this the independent approach.
Short Term Parking
A typical configuration for short-term parking consists of a simple two-way, two-position valve, which is installed between the brake control valve and the brakes.
A check valve opens and full metered pressure is applied to the brake. When the pilot releases the brake pedal, the parking brake valve traps the pressure at the brake. If the trapped brake pressure exceeds the normal working pressure of the system (because of thermal effects such as heat from the brakes soaking back through the pistons) the relief device bleeds off this excessive pressure. As the brakes cool down, the pressure can fall below that required to hold the airplane limiting the holding time provided by this method. This approach will typically maintain required pressure for 30 minutes or so.